The photosynthetic reduction of carbon dioxide depends on the availability of both adenosine triphosphate ( ATP ) and reduced triphosphopyridine nucleotide ( NADPH ), which in chloroplasts are formed at the expense of light energy (Trebst, Tsujimoto & Amon 1958). Under anaerobic conditions, isolated chloroplasts are capable of synthesizing ATP from inorganic phosphate and ADP in light, provided they are supplemented with catalytic amounts of an electron carrier such as vitamin K3, flavin mono-nucleotide ( FMN ), phenazine methosulphate ( PMS ) or 2,4-dinitrophenol ( DNP ) (Amon, Whatley & Allen 1954,1955; Whatley, Allen & Amon 1955; Amon, Allen & Whatley 1956; Jagendorf & Avron 1958; Wessels 1959 b; Arnon, Losada, Whatley, Tsujimoto, Hall & Horton 1961). This ATP formation is named cyclic photophosphorylation to distinguish it from the (non-cyclic) photophosphorylation associated with the reduction of substrate amounts of NADP or K 3 Fe(CN) 6 (Arnon, Whatley & Allen 1958, 1959), and from the phosphorylation occurring under aerobic conditions (Arnon et al. 1961; Wessels 1958; Jagendorf & Avron 1959; Nakamoto, Krogmann & Vennesland 1959; Trebst & Eck 1961a). A direct reduction of pyridine nucleotides by illuminated chloroplasts was first demonstrated by San Pietro & Lang (1956). A water-soluble protein, called pyridine nucleotide reductase, was required for NADP reduction (San Pietro & Lang 1958), but not for the photochemical reduction of ferricyanide or quinone (Amon et al. 1958, 1959).